This invention relates to an apparatus for protecting a spot network type power receiving and transforming system, and more particularly to improvements in such a protective apparatus so that its protecting function can be reliably performed even when overall service interruption occurs at the primary side of network transformers during parallel operation of a spot network type power receiving and transforming system and an electric generator system.
An apparatus for protecting a spot network type power receiving and transforming system is widely employed as means for protecting a power receiving and transforming system in buildings because of its high reliability of interruption-free supply of power.
FIG. 1 is a diagram showing a basic structure of a prior art apparatus for protecting a spot network type power receiving and transforming system, and such a protective apparatus is disclosed in, for example, JP-A-57-68624. FIG. 1 illustrates the flowing direction of short-circuit current in the event of a short-circuit trouble in the spot network type power receiving and transforming system.
Referring to FIG. 1, a plurality of branch buses B.sub.1 to B.sub.3 are connected between three-phase power supply buses L.sub.1 to L.sub.3 and a common bus B.sub.0 to which a plurality of loads R including, for example, an elevator M are connected. Network transformers T.sub.1 to T.sub.3, protector circuit breakers P.sub.1 to P.sub.3 and current transformers C.sub.1 to C.sub.3 are connected to the branch buses B.sub.1 to B.sub.3 respectively. When a short-circuit trouble occurs at, for example, a point A of the A L.sub.1 power supply bus, a short-circuit current i.sub.s as shown by the solid lines in FIG. 1 flows toward the shorted point A through the route of the sound B L.sub.2 power supply bus - branch bus B.sub.2 - common bus B.sub.0 - branch bus B.sub.1. In the branch bus B.sub.1, this short-circuit current i.sub.s flows in a reverse direction. That is, reverse current flows from the common bus B.sub.0 toward the L.sub.1 power supply bus and is detected by the current transformer C.sub.1. The detection output current of the current transformer C.sub.1 actuates a network relay N.sub.1, and the protector circuit breaker P.sub.1 is tripped to protect the spot network type power receiving and transforming system. This arrangement is generally called a spot network protector system. Considering an emergency in which supply of power from the power supply buses is interrupted, an emergency electric generator is disposed on one side of the common bus B.sub.0, so that required power can be supplied to an emergency system only when supply of power to the power receiving system is entirely interrupted.
Recently, a multipurpose power generation system (referred to hereinafter as a co-generation system) has been put into practical use. In this co-generation system, electric power is generated by an electric generator driven by, for example, a gas turbine or a gas engine, and, at the same time, the waste heat included in the exhaust from the gas turbine or gas engine is utilized for supplying, for example, hot water. As an energy saving system, this co-generation system finds its wide applications. Also, this co-generation system is also employed in buildings of medium and large scales. From the aspects of the effective utilization of the co-generation, and of the prevention of instantaneous break of a load this co-generation system is required to be temporarily or continuously linked with a power receiving system receiving power supplied from an electric power company. For the reasons described already, a spot network type power receiving and transforming system is widely employed in buildings. Because the co-generation system is also now being widely employed in buildings together with the spot network type power receiving and transforming system, parallel operation of the spot network type power receiving and transforming system and the electric generator of the co-generation system is now demanded.
When a short-circuit trouble occurs at a point of one of the primary-side power supply buses of the network transformers in the spot network type power receiving and transforming system, the short-circuit point must be isolated as soon as possible, and power distribution by the remaining sound power supply buses must be continued, as described already. For this purpose, each network transformer bank in the spot network protector system has a reverse power interrupting function. That is, a current from the network transformers connected to the sound power supply buses flows in a reverse direction through the network branch buses toward the network transformer connected to the faulty power supply bus, and the spot network protector system operates by detecting this reverse current thereby isolating the short-circuit point.
In the spot network type power receiving and transforming system, a situation has frequently occurred where regenerative power regenerated from the motor of the elevator produces flow of reverse power in the system even when no electric generator is provided on the load side. Such a situation is attributable to the fact that, when the elevator moves downward with full load or moves upward with no load, power substantially equal to that required for moving the elevator upward under full load is fed back toward the power supply side. When the total load of the power receiving and transforming system is large, the regenerative power regenerated from the motor of the elevator is canceled by the power received from the power supply, and the spot network protector system is not adversely affected. However, the total load of the power receiving and transforming system is small as in the nighttime, the regenerative power tends to become larger than the received power, and extra power is fed back toward the power supply side.
FIG. 2 shows that regenerative current from the motor of the elevator flows in the prior art spot network type power receiving and transforming system shown in FIG. 1. When the elevator M is of the gearless type having no reduction gearing, and the elevator M moves downward with full load or moves upward with no load, power substantially equal to that required for moving the elevator upward under full load is fed back toward the power supply side. Regenerative current i.sub.M produced by the regenerative power regenerated from the motor of the elevator M is shown by the dotted lines in FIG. 2. When the total load of the power receiving and transforming system is large as in the daytime, the regenerative power is canceled by the power received from the power supply, and the received power is only slightly decreased. However, in the nighttime in which the total load is small, extra power is fed back toward the power supply side from the network buses through the network transformers. Therefore, a controller (not shown) will decide that reverse power appears in all the network buses, and the spot network protector system will act to interrupt supply of power from the power supply buses, resulting in overall service interruption. Thus, when reverse power is supplied to all the banks, the controller decides that the appearance of reverse power is attributable to regenerative power regenerated from the motor of the elevator, and the spot network system is locked so as to prevent mal-operation of the circuit breakers.
When overall service interruption occurs on the primary-side power supply buses of the network transformers while the electric generator of, for example, the co-generation system is operating in parallel with the spot network type power receiving and transforming system, a reverse voltage is applied to the power supply buses from the electric generator. In such a case, all the network transformers are excited in the reverse direction, and reverse current flows in all the network transformers. As a result, the controller decides that regenerative power is supplied from the motor of the elevator, and the spot network protector system is locked as described above. Due to the locking of the spot network protector system, the power supply buses will be charged even in the case of service interruption. Such a situation is very dangerous from the aspect of safety of maintenance and inspection of the buses.